Strikes me that the choice is straightforward:
1. if you use an ioctl, software (e.g. dump, tar, mt) must be modified to
take advantage.
2. if you use a bit in the minor device for a differentiated device node,
all existing software works as before (modulo default device name
selection).
I presume this is principally an issue when writing, and that the drives in
question can auto-detect when reading? If not, you'd better be careful when
labelling your tapes.
Erik <fair@clock.org>